Magnetic resonance imaging (MRI) is a medical diagnostic imaging technique used to diagnose many types of injuries and medical conditions. An MRI system includes a main magnet for generating a main magnetic field through an examination region. The main magnet is arranged such that its geometry defines the examination region. The orientation of the main magnet defines whether the MRI system is classified as a horizontal field system or a vertical field system. In a vertical field system, the static magnetic field is typically oriented in an anterior-posterior (A-P) direction relative to the prone/supine patient within the system. In a horizontal field system, the static main magnetic field is typically oriented in the head-foot (H-F) direction relative to the prone/supine patient within the system.
The main magnetic field causes the magnetic moments of a small majority of the various nuclei within the body to be aligned in a parallel or anti-parallel arrangement. The aligned magnetic moments rotate around the equilibrium axis with a frequency that is characteristic for the nuclei to be imaged. An external radiofrequency (RF) field applied by other hardware within the MRI system perturbs the magnetization from its equilibrium state. Upon termination of the application of the RF pulse, the magnetization relaxes to its initial state. During relaxation the time varying magnetic moment induces a detectable time varying voltage in the receive coil. The time varying voltage is commonly detected by a RF receive coil.
During operation of the RF receive coil, each element within the coil collects information from the time varying voltage induced by the magnetic moments within the anatomy of the patient nearest to that element. The information collected by each element is processed through the electronics within the MRI system on individual channels of the MRI system, which keep the information from each element separate throughout the imaging process. The information from each channel of the system is then processed by reconstruction software integrated with the MRI system to combine the single images from the channels to create a complete image of the anatomy of interest.
One or more RF receive coils, commonly called imaging coils, are typically placed within the vicinity of the patient during imaging. The imaging coil is typically comprised of a series of inductive and capacitive elements and operates by resonating and efficiently storing energy at what is known as the Larmor frequency. The imaging coil is comprised of at least one, and usually more than one element typically made of a continuous piece of copper in a solenoid, loop, butterfly or figure-eight (saddle), or other continuous geometric shape. The elements are positioned at various locations throughout coil to provide for the desired imaging of the patient. The design of the receive coil varies depending on whether it is designed for use within a vertical or horizontal field MRI system.
The shape, configuration and location of elements within the receive coil affect the characteristics of the coil, including the coil sensitivity, signal-to-noise ratio (SNR) and imaging field-of-view. Conventionally, the receive coil's imaging field-of-view (FoV) is defined as the distance between the two points on the coil sensitivity profile, which is a graph of the coil's sensitivity over the distance profile, where the signal drops to 80% of its peak value. The shape and design of the RF receive coil varies depending on the patient anatomy the coil is designed to imagine.
Further developments in MRI include various parallel imaging techniques. An example of a parallel imaging technique is Simultaneous Acquisition of Spatial Harmonics (SMASH). The SMASH technique uses a parallel processing algorithm to exploit spatial information inherent in a surface coil array. The result is an increase in MR image acquisition speed, resolution and/or field of view. In a similar fashion, another parallel processing algorithm is known where the acceleration of image acquisition is performed on the time domain space instead of the frequency domain space. This parallel acquisition technique is referred to as Sensitivity Encoding (SENSE). In SENSE, images are obtained by means of magnetic resonance (MR) of an object placed in a static magnetic field and includes simultaneous measurement of a number of sets of MR signals by application gradients and an array of receiver coils. The characteristics of all of these parallel imaging techniques is that the acceleration speed is directly proportional to the number of independent receivers along the direction that the image acceleration needs to be applied. Thus, the higher the number of receiver coils, the faster the acceleration speed for acquiring an image with better SNR and improved image quality.
A human shoulder is one of the many types of patient anatomy that is imaged using MRI technology. Imaging a human shoulder includes the visualization of the various parts of the shoulder, including the glenoid fossa, labrum, humeral head, neck and body, supraspinatus, infraspinatus and teres minor insertion (rotator cuff) and surrounding soft tissues. MRI examinations of the shoulder region are often done to visualize rotator cuff tears, labral tears, labral deterioration, and impingement syndrome, among other reasons.
Within the art, numerous attempts have been made to provide designs for RF receive coils for shoulder imaging in a vertical field MRI system. Designs of the prior art commonly use linear loop coils. The linear coils have a strong signal at the center of the loop, but the coverage, uniformity and penetration are too poor for adequate clinical application, even at the center of the region of interest for the human shoulder. Furthermore, multi-purpose coils are commonly used for shoulder imaging. This is not ideal because the coil is not designed to give optimal imaging for the shoulder region.